Nucleoside analogues are highly effective agents for treatment of viral infectious diseases such as AIDS, hepatitis B, herpes virus, herpes zoster, cytomegalovirus and the like. A large number of significant known compounds include Entecavir, Abacavir, Lamivudine, Tenofovir, Adefovir, Acyclovir, Ganciclovir, Famciclovir, Lobucavir and the like. Nucleoside analogues, such as Gemcitabine, Cladribine (2-CdA), Fludarabine, Clofarabine, Nelarabine and the like, are effective for treatment of neoplastic diseases. It is expected that nucleoside analogues should also play a pivotal role in anti-HCV and anti-DENV treatment with one recently approved nucleotide (Sofosbuvir) of this kind and several nucleoside analogues in late stage clinical developments at the present time.
Most of anticancer and antiviral nucleoside analogs require metabolic activation to the 5′-mono, di-, and triphosphates via sequential phosphorylation by nucleoside and nucleotide kinases. Inefficient metabolic activation has been known to be among causes of the lack of therapeutic effectiveness of many nucleoside analogs, and as an important mechanism of nucleoside drug resistance. In these cases, usually the first phosphorylation step is rate limiting, thus the activity of these nucleosides can be rescued by various phosphate and phosphonate prodrug strategies. A variety of nucleoside monophosphate prodrugs (pronucleotides or ProTides), such as aryloxy phosphoramidate diesters, bisPOM, cycloSAL, hepDirect, and SATE, have been developed to improve their therapeutic activity by increasing the intracellular uptake of mono-phosphorylated nucleoside drugs. Prominent examples include Adefovir dipivoxil, Tenofovir disoproxil, and Sofosbuvir (GS-7977). However, existing ProTides have issues such as intracellular release of toxic agents (aryl alcohols, aryl vinyl ketone, ethylene sulfide, etc.), poor stability, varieties in intra- and interpatient pharmacokinetic and pharmacodynamic profiles, and low yields in synthesis. As an example, INX-189 produces intracellularly 1-naphthol, which, in addition to postulated mitochondrial toxicity of the resulting nucleotide, potentially caused the failure of this compound. As another example, Sofosbuvir, a clinical anti-HCV drug, is an aryloxy phosphoramidate diester and releases toxic phenol.
3′,5′-Cyclic phosphates and phosphoramidates of nucleotides have been developed as uncommon prodrug structures to deliver nucleoside 5′-phosphates into cells with the expectation of improved cellular uptake by reducing the rotational degrees of freedom with a conformationally constrained structure and blocking the 3′-hydroxyl group to reduce polarity, and also removing toxic phenol or 1-naphthol. A variety of derived prodrug forms include phosphoramidate, SATE, pivaloyloxymethyl (POM), and simple alkyl ester groups as substituents of such 3′,5′-cyclic phosphates and phosphoramidates. One notable example is PSI-352938, a suspended investigational drug for anti-HCV treatment because of the observed elevated level of liver enzymes in patients in clinical trials. It was reported that PSI-352938 was activated to nucleoside 5′-phosphates via a key step of selective ring opening by phosphodiester cleavage of the 3′-phosphate-oxygen bond catalyzed by cyclic nucleotide phosphodiesterase (PDE) enzymes. We believe this PDE-mediated hydrolytic activation be shared by all 3′,5′-cyclic phosphate and phosphoramidate prodrugs, may impact the tightly regulated cellular cyclic nucleotide signaling pathways, which are temporally, spatially, and functionally compartmentalized, and result in abnormal intracellular concentrations of cyclic nucleotides (such as cAMP and cGMP), and, consequently, myriad biological responses leading to human diseases. These cyclic nucleotides may also act as secondary messengers and lead to undesired physiological changes, with their median effective concentrations (EC50) in or close to the range of apparent activation constants (Ka) of protein kinase A (PKA RIβ2:C2, 29 nm; RIα2:C2, 101 nm; RIIα2:C2, 137 nm; and RIIβ2:C2, 584 nm) for cAMP and of protein kinase G (PKG-Iα, 67 nm and PKG-Iβ, 133 nm) for cGMP.
Therefore, there are needs for better pronucleotides. To minimize cytotoxicity, properties of tissue targeting (i.e. concentration of active drugs in liver tissues based on ProTide strategy), no release of cytotoxic metabolites, and clearance of disrupting nontargeted vital cellular/biological processes (off-target effects) by the prodrug and its intermediates are extremely attractive.